1. Field of the Invention
The present invention relates in general to electronic circuits and in particular to circuits and methods for controlling transients during audio device power-down, and systems using the same.
2. Description of the Related Art
Purchasers of state of the art home and portable audio systems expect improved audio performance, as well as more options for controlling playback from the given recording media. One of the most important performance criteria is the elimination of clicks, pops, noise and other artifacts audible to the user. Not only are these audible artifacts distracting to the listener, but they can also damage the system speakers or headset. This is especially true with transient artifacts, such as clicks and pops, which may spike the output signal driving the speakers or headset to a relatively high level.
Clicks and pops can be caused by a number of different conditions, such as device power-down or power-up, steps in the output signal under digital volume control, and similar situations where the output signal voltage transitions at a high rate. While there are presently a number of different of techniques for controlling clicks pops, these techniques adversely impact other aspects of system performance. Consequently, new circuits and methods are needed which control clicks, pops without significantly impacting other aspects of system performance.
The principles of the present invention are embodied in circuits and methods for driving an output load which do not generate transients if the power supply to the driver is interrupted. According to one embodiment, a driver circuit is disclosed which includes a p-channel transistor for driving an output from a supply rail at a positive supply voltage, the p-channel transistor disposed in an n-well. A detector detects ramp down of the supply voltage below a preselected threshold voltage and a power reservoir maintains a preselected well voltage of the n-well after the supply voltage ramps down below the preselected threshold.
By maintaining the well voltage for at least the p-channel transistors in the output driver, potential leakage paths between the output load and the power supply rail on power down are cut-off. Specifically, the n-wells, in which the p-channel transistors are fabricated, are kept at a voltage sufficient to ensure that the source to n-well and drain to n-well diodes of those transistors are reverse biased.